1. Field of the Invention
The present invention relates to a lubricant adapted for use in hot work and, more particularly, to a lubricant which is suitably applied to the surfaces of hot-work tools such as a plug, guide shoe and so forth employed in a process for producing a seamless steel pipe through rolling by a cross rolling mill.
2. Description of the Related art
In general, a process for producing a seamless steel pipe in accordance with the Mannesmann method basically has the steps of: (1) piercing a round billet to form a hollow bloom; (2) elongating the hollow bloom to elongate the same; and (c) finish-rolling the tube. The steps including the piercing, elongating and finish rolling are performed with the help of tools and devices such as plugs, guide shoes, rolls and so forth.
Among these tools and devices, plugs are easily worn. Reducing the wear of the plugs is beneficial from the view point of efficiency, economy and product quality in the pipe production process. The surface of the plug is covered by a thick oxide scale which is closely adhered to the base material. The scale serves as a heat insulating layer for protecting the plug body, thus directly affecting the life of the plug. In addition, the scale reduces the rolling load and prevents deterioration. The scale alone, however, cannot provide satisfactory protective effect in many instances. Attempts have been made, therefore, to lubricate plugs used for directly piercing billets, by applying lubricants to the plugs.
For instance, Japanese Unexamined Patent Publication Nos. 51-57729 and 1-180712 disclose methods in which an oily lubricant is sprayed from the end of the plug. Japanese Unexamined Patent Publication No. 5-138213 discloses a method in which a graphite-type lubricant is applied to the plug surface before the plug is used for piercing. The first-mentioned method, however, has not yet been successfully introduced to the industry, because of difficulty encountered in forming a spray nozzle on the plug's head without impairing the shape of the plug's head which is an important factor of the plug design in the piercing process. The second-mentioned method also suffers from a disadvantage in that the graphite allows slippage of the plug. Thus, the graphite does not fully contribute to the improvement in the piercing efficiency but, rather, involves a risk of allowing problems such as failure in biting the material to be rolled and failure in the sticking of the tail end of the rolled material from the roll.
The conditions under which the plugs are used are becoming more severe, due to the current tendency towards the use of stainless steels and alloy steels to form seamless steel pipes. This is because alloy steels pose higher levels of piercing loads than ordinary steels. More specifically, when an alloy steel is used as the pipe material, the scale on the plug surface is exfoliated in a short time due to the heavy piercing load, so that the plug directly acts on the material subjected to rolling without an intermediate layer which would serve as a heat-insulating and lubricating layer, with the result that the wear of the plug is promoted.
In the production of a seamless steel pipe by a Mannesmann-type piercing mill, the pipe material is rolled and pierced by means of a pair of opposing skews or cross rolls and a plug. At the same time, a pair of guide shoes are used to prevent the outside diameter of the rolled material from increasing due to the rolling. The guide shoes may be of a stationary type or of a disk-roll type.
The surfaces of the guide shoes are in such a state as to permit easy slip of the guide shoes with respect to the surface of the rolled material in the circumferential direction of the material. In addition, the guide shoes which act to prevent radial expansion of the material have to sustain a large reaction force. Consequently, the surfaces of the material tend to adhere to the guide shoes surface subjected to rolling, particularly when the material to be rolled is a high-alloy steel represented by about 13% Cr steel, about 22% Cr steel or stainless steel.
As measures for preventing such score, Japanese Unexamined Patent Publication No. 60-56406 and Japanese Examined Patent Publication No. 5-16925 disclose, respectively, methods for rolling while supplying the guide shoe surface with graphite-type lubricant and a boric acid type lubricant. In both methods, application of the lubricant to the guide shoe surface is performed by spraying.
Each of these lubricating methods, however, suffers from a problem in that flaws are liable to be generated due to insufficient anti-scoring effect when the rate of supply of the lubricant is too small. Conversely, excessive supply of the lubricant tends to cause a rolling failure due to slip between the rolled material and the rolls which undesirably reduces the friction coefficient, particularly when the lubricant is of the graphite type as disclosed in Japanese Unexamined Patent Publication No. 60-56406. When the lubricant is of the boric acid type as proposed in Japanese Examined Patent Publication No. 5-16925, the lubricant tends to be washed away by a large quantity of water such as that used for cooling, thus impairing the anti-scoring effect.
Japanese Unexamined Patent Publication No. 6-142749 discloses a method in which a billet is rolled while its surface is being supplied with a lubricant of the sodium silicate type. Japanese Unexamined Patent Publication No. 7-116709 discloses a method in which rolling is conducted while supplying the roll surface with a lubricant of swelled mica-type lubricant.
These methods, however, have the following shortcomings. The method disclosed in Japanese Unexamined Patent Publication No. 6-142749, which relies upon the application of a sodium silicate type lubricant on the billet surface during rolling, allows the exfoliation of the scale from the rolled material during transportation or rolling of the billet, as well as the separation of the lubricant from the material surface, thereby failing to provide appreciable anti-scoring effect. The method disclosed in Japanese Unexamined Patent Publication No. 7-116709, which relies upon the application of a swelled mica-type lubricant on the roll surface during rolling, is also liable to fail to satisfactorily prevent seizure because the lubricant tends to be washed away by the supply of a large quantity of water such as roll cooling water and the separation of lubricant due to exfoliation of the scale from the surface of the material subjected to the rolling.
Japanese Unexamined Patent Publication No. 5-148493 discloses the use of an aqueous solution of sodium silicate containing graphite or mica as a lubricant for lubricating a rolled material. This water-glass type lubricant, when heated, foams by allowing water content to evaporate therefrom and becomes a pumice-like substance which is then vitrified when the temperature reaches a softening point. When this type of lubricant is applied to the outer surface of the hot material to be rolled, the lubricant is softened into a glassy state so as to produce a lubricating effect upon contact with the guide shoes and rolls. This type of lubricant, by virtue of its liquid nature, can easily be applied to plugs and guide shoes which are normally held at comparatively low temperatures and, hence, seems to be suitably usable as a lubricant for such plugs and guide shoes. The inventors have found, however, that this type of lubricant cannot exhibit appreciable lubricant effect when applied to the plugs and guide shoes, for the reason that the lubricant when so used is subjected to a shearing load before it is heated up to the softening point at which it becomes glassy so that it easily comes off the surface of the plug or the guide shoe.
Japanese Unexamined Patent Publication No. 5-171165 discloses a lubricant for a material to be rolled. This lubricant has a composition composed of a particulate oxide-type laminar compound and a binder containing an alkali borate mixed with boron oxide and boric acid. Unfortunately, this lubricant does not exhibit a liquid state and, hence, cannot exhibit satisfactory adhesion or spreading by melting on the surface of a plug or guide shoes, which are usually cooled to temperatures much lower than that of the rolled material, which is usually at an elevated temperature of 900.degree. C. or higher at the internal hollow surface.
Thus, in the field of production of seamless steel pipes by the Mannesmann method, it is desirable to decrease the coefficient of friction between the material subjected to rolling and the tools such as the plug and guide shoes which are used in the step of piercing a billet and the subsequent elongating and rolling steps, thereby prolonging the lives of the plug and the guide shoes. This is particularly desirable in the cases where the billet to be processed is made of a material which imposes a heavy load on the rolling tool and other devices, such as a stainless steel or an alloy steel, is used as the material of the billet.